Enhance PR template with description and context

Added sections for 'Description' and 'Context' in PR template.

Author: chrisbloe

.github/PULL_REQUEST_TEMPLATE.md

@@ -3,9 +3,13 @@
 
 **Jira ticket**: [TBC](https://nhsd-jira.digital.nhs.uk/browse/XXX)
 
-**Description**: TBC
+### Description
 
-**Context**: TBC
+<!-- Describe your changes in detail. -->
+
+### Context
+
+<!-- Why is this change required? What problem does it solve? -->
 
 ## Checklist